The invention relates to a method and a fibre-optical system for illuminating and detecting an object by means of light, in particular for endoscopic and microscopic applications. The invention comprises in situ calibration of fibre-optical image waveguides (fibre bundles) and disturbance correction of illumination or detection.
Fibre-optical systems in the form of flexible endoscopes comprise a high number (up to several 100,000) of individual, flexible optical fibres, e.g. glass fibres. If the individual fibres of a fibre bundle are arranged coherently, optical image transmission can be carried out. Advantages compared with rigid endoscopes, in which the image information is transferred via a rod lens system, are in particular the compactness of the extremely fine fibre bundle and the flexible use possibilities, even when access to the observed object is extremely restricted. A disadvantage of flexible endoscopes is that the achievable image contrast is relatively low. This is caused in particular by the pixelation of the fibre bundle, the crosstalk between the individual fibres, leading to a reduction in the image sharpness (blur effect), and multi-mode nature of the light wave guidance in the individual fibres (speckle effect). The deterioration in the image quality owing to the described disturbances from the fibre bundle can be described by means of a modulation transfer function. Furthermore, flexible fibre bundles only allow for transmission of intensity profiles and the imaging thereof using lenses (near-field technology). Owing to the different and unknown optical path lengths of the individual fibre, the phase information of the transmitted light is not directly accessible, and therefore the far field of the transmitted light is unknown and cannot be specifically adjusted.
A reduction in the pixel spacing of an image made using a flexible endoscope can be achieved by integrating imaging optics, generally an optical lens or a lens system which images the distal plane of the fibre bundle onto the object plane. However, this significantly increases the installation size of the endoscope, and therefore the minimum dimension for optical access is increased. Moreover, complex construction and connection techniques are required for integrating the imaging optics, as well as numerous adjustment steps. A further disadvantage results from the fact that fibre bundles comprising conventional imaging optics allow for only two-dimensional measurements in the lateral plane. In order to obtain depth information, for example scanning methods have to be carried out using mechanically displaceable optical elements or electrically adjustable optics, or other complex measurement methods (triangulation using two fibre bundles).
Various approaches exist for correcting disturbances caused by transmission through optical waveguides. WO 2004/032386 A1 describes a method for correcting polarisation-dependent disturbances in light waveguides, a compensation function for the polarisation-dependent disturbances being calculated by the light waveguide, by means of which function a distorted electrical input signal is first calculated, which signal is converted into a corresponding distorted optical input signal which is intended to be transmitted via the light waveguide. In order to determine the compensation function, a detector is used which is configured for measuring the signal-noise ratio, the polarisation-dependent attenuation or mode dispersion, the bit error rate, or the signal dispersion.
Adaptive optics methods are known from the prior art, in which phase aberrations can be detected and equalised by using wavefront sensors and modulators. Methods of this kind for beam control of high-energy laser beams are disclosed for example in DE 602 23 130 T2. Cui, M. and Yang, C. Implementation of a digital optical phase conjugation system and its application to study the robustness of turbidity suppression by phase conjugation. Optics Express 18, Vol. 4 (2010), page 3444 proposes an adaptive optics method comprising an open control loop that is referred to as “digital optical phase conjugation” and is suitable for correcting phase distortions through an optically opaque, in particular biological, medium. In this case, a CCD camera is used as a sensor and a spatial light modulator (SLM) is used as an actuator. The described method requires each pixel of the camera to generate a virtual image on a corresponding pixel of the SLM, and vice versa. As a result, the method requires complex calibration and a very high degree of adjustment work. Gu, R. Y., Mahalati, R. N. and Kahn, J. M. Design of flexible multi-mode fiber endoscope. Optics Express 23, Vol. 21 (2015), page 26905 describes an endoscope comprising flexible multi-mode fibres which can be calibrated using a partial reflector at the distal end and an SLM at the proximal end. Calibration of the endoscope and measurements using the endoscope cannot be carried out simultaneously. The specifications “proximal” and “distal” characterise positional relationships with respect to the fibre bundle, the proximal side of the fibre bundle being the end face that faces the illumination source, and the distal side of the fibre bundle being the end face that faces the object.
U.S. Pat. No. 8,585,587 B2 described a flexible endoscope in which the relative phase of the incident light can be changed using an SLM arranged at the proximal fibre end. The phase difference caused by the fibres with respect to the incident light is determined by means of a wavefront sensor or interferometer. For this purpose, a partially reflective coating is applied to the distal fibre ends. As a result, the phase difference Δϕ is determined after the light has passed through the fibre bundle twice, and therefore it is not the single phase difference that is measured but instead 2Δϕ, modulo 2π in each case. The disadvantage of measuring the double phase difference is that this measurement is unclear. For example, if a double phase difference about π is measured, the single phase difference caused by the fibre bundle may be π/2 or −π/2. The true single phase difference can be determined only using complex methods.
In order to overcome this, US 2015/0015879 A1 proposes distally illuminating a multi-mode waveguide by means of a virtual light source image. A multi-mode fibre that has a double sheathing and comprises a single-mode fibre core is considered, at the distal end of which fibre a holographic photographic material and a point-reflection generating object (by which a virtual light source is generated behind the object) are positioned. In the event of illumination, the wave transmitted by the single-mode fibre interferes constructively, at the location of the photographic material, with the light emanating from the virtual light source. It is disadvantageous that the position of the virtual light source is fixed and cannot be freely selected, and therefore the position selection for the real focus is also greatly restricted. Moreover, a specific optical design is necessary at the distal fibre end.